1. Field of the Invention
The present invention relates to an image forming apparatus equipped with a sheet conveying device for conveying a sheet-like recording medium on which an image is formed.
2. Related Background Art
A conventional image forming apparatus is constructed as shown in, e.g., FIG. 17.
Referring to FIG. 17, record sheets defined as sheet-like recording mediums singly fed by a sheet feeding roller 11 out of a sheet supply cassette 10, are conveyed via a convey passageway A to a regist roller 12. When a front edge of a developed image formed on a photosensitive drum 13 arrives at a position of a transfer nip portion at which the photosensitive drum 13 is brought into contact with a transfer roller 14, a timing is taken so that the front edge of the record sheet is conveyed to the transfer nip portion by controlling rotations of the regist roller 12. Then, an image forming portion is constructed of the photosensitive drum 13 and the transfer roller 14.
The developed image on the photosensitive drum 13 is thus transferred onto the record sheet at the transfer nip portion, and thereafter the record sheet is separated from the photosensitive drum 13 and conveyed through a convey belt 15 to a fixing device 16. In this fixing device 16, the record sheet is sandwiched by pressure in between a heating member 17 and a pressurizing roller 18, and an unfixed image transferred onto the record sheet is fixed. Thereafter, the record sheet onto which the toner image has been fixed is discharged a couple of sheet discharge roller 19 via a convey passageway B to a tray 20 of a main body 1 of an image forming apparatus.
Incidentally, the couple of sheet discharge rollers 19 is constructed of a driving roller 19a composed of rubber, a driven roller 19b composed of resin which is rotatably supported by a holder 19c, and a spring 19d for pressing the driven roller 19b against the driving roller 19a.
The driven roller 19b of the couple of sheet discharge rollers 19 is, however, so constructed as to be pressed by a spring 19d into contact with the driving roller 19a, and rotationally driven by a frictional resistance against the record sheet. Therefore, if a frictional resistance (a conveying force) between the record sheet and the driven roller 19b is small, the driven roller 19b does not rotate, and consequently the problems given hereunder might arise.
1) If the frictional resistance between the record sheet and the driven roller 19b is small, the driven roller 19b is composed of a resinous material and is therefore unable to rotate due to its slippage. This might cause problems in which a convey deterioration such as a deterioration in discharging the sheet occurs, and the record sheets are not neatly staked on the tray 20.
2) Since the driven roller 19b does not rotate, it might happen that a toner image (a developer image) on the record sheet is scratched off.
3) Because of the driven roller 19b not smoothly rotating, the toners are adhered to the driven roller 19b, so that the toners on the driven roller 19b are then adhered to the record sheet when in a next image forming process, resulting in a state of being unable to obtain a well-formed record image.